Cotrimoxazole – components and therapeutic rationale MCQs With Answer

Cotrimoxazole – components and therapeutic rationale MCQs With Answer
Cotrimoxazole is a fixed-dose antimicrobial combining sulfamethoxazole and trimethoprim that produces synergistic inhibition of folic acid synthesis. For B. Pharm students, mastering its components, mechanism (DHPS and DHFR inhibition), pharmacokinetics, spectrum (UTI, PCP, Nocardia, some enteric pathogens), resistance mechanisms, dosing ratios, adverse effects (hypersensitivity, bone marrow suppression, hyperkalemia), and key drug interactions is essential. Understanding therapeutic rationale explains why the combo is bactericidal and how clinical decisions balance efficacy and toxicity. This set emphasizes clinical pharmacology, safety, and formulation considerations. Now let’s test your knowledge with 30 MCQs on this topic.

Q1. Which two enzymes in the folate synthesis pathway are sequentially inhibited by cotrimoxazole components?

• Dihydropteroate synthase and dihydrofolate reductase
• Dihydrofolate synthase and thymidylate synthase
• Dihydropteroate synthase and DNA gyrase
• Dihydrofolate reductase and RNA polymerase

Correct Answer: Dihydropteroate synthase and dihydrofolate reductase

Q2. What is the usual weight ratio of trimethoprim to sulfamethoxazole in standard cotrimoxazole formulations?

• 1:5 (trimethoprim : sulfamethoxazole)
• 5:1 (trimethoprim : sulfamethoxazole)
• 1:1
• 2:5 (trimethoprim : sulfamethoxazole)

Correct Answer: 1:5 (trimethoprim : sulfamethoxazole)

Q3. Which of the following best explains the synergistic bactericidal action of cotrimoxazole?

• Concurrent blockade at two consecutive steps in folate synthesis leads to sequential inhibition and bactericidal effect
• Both drugs inhibit protein synthesis at the 30S ribosomal subunit
• The combination increases bacterial cell wall permeability causing lysis
• One drug neutralizes bacterial toxins while the other inhibits DNA replication

Correct Answer: Concurrent blockade at two consecutive steps in folate synthesis leads to sequential inhibition and bactericidal effect

Q4. Which organism is a classical non-bacterial pathogen effectively treated by cotrimoxazole?

• Pneumocystis jirovecii
• Aspergillus fumigatus
• Candida albicans
• Cryptococcus neoformans

Correct Answer: Pneumocystis jirovecii

Q5. A major hematologic adverse effect of prolonged cotrimoxazole use is:

• Bone marrow suppression leading to megaloblastic anemia
• Aplastic anemia from immediate allergic reaction
• Hemolytic anemia due to iron deficiency
• Polycythemia vera-like erythrocytosis

Correct Answer: Bone marrow suppression leading to megaloblastic anemia

Q6. Which renal tubular effect is attributable primarily to trimethoprim?

• Hyperkalemia due to inhibition of potassium secretion in the collecting tubule
• Hypokalemia due to increased distal sodium exchange
• Polyuria due to nephrogenic diabetes insipidus
• Metabolic alkalosis from enhanced bicarbonate reabsorption

Correct Answer: Hyperkalemia due to inhibition of potassium secretion in the collecting tubule

Q7. The most important mechanism of bacterial resistance to sulfamethoxazole involves:

• Plasmid-encoded altered dihydropteroate synthase with reduced drug binding
• Mutations in ribosomal RNA preventing drug binding
• Increased production of DNA gyrase
• Enzymatic hydrolysis of trimethoprim

Correct Answer: Plasmid-encoded altered dihydropteroate synthase with reduced drug binding

Q8. Which pharmacokinetic property of trimethoprim contributes to its usefulness in treating urinary and intracellular infections?

• Good oral absorption and extensive tissue penetration including intracellular and renal tissues
• Poor oral absorption but high urinary excretion only
• High plasma protein binding preventing tissue penetration
• Rapid hepatic metabolism leading to short duration of action

Correct Answer: Good oral absorption and extensive tissue penetration including intracellular and renal tissues

Q9. Cotrimoxazole is contraindicated in late pregnancy primarily because of:

• Risk of neonatal kernicterus and interference with folate metabolism
• Teratogenic cardiac malformations in the second trimester only
• Induction of premature labor via prostaglandin release
• High risk of maternal hypoglycemia

Correct Answer: Risk of neonatal kernicterus and interference with folate metabolism

Q10. Which laboratory parameter should be regularly monitored during long-term cotrimoxazole therapy?

• Complete blood count to detect bone marrow suppression
• Serum amylase for pancreatitis
• Fasting blood glucose for hyperglycemia
• Thyroid function tests for hypothyroidism

Correct Answer: Complete blood count to detect bone marrow suppression

Q11. A clinically important drug interaction between cotrimoxazole and warfarin results in:

• Increased anticoagulant effect due to inhibited warfarin metabolism
• Decreased anticoagulant effect due to enzyme induction
• No interaction because cotrimoxazole is renally excreted
• Increased protein binding of warfarin reducing bleeding risk

Correct Answer: Increased anticoagulant effect due to inhibited warfarin metabolism

Q12. Which adverse reaction is most associated with the sulfonamide component of cotrimoxazole?

• Stevens-Johnson syndrome and hypersensitivity reactions
• Serotonin syndrome
• Tendon rupture
• Otitis media

Correct Answer: Stevens-Johnson syndrome and hypersensitivity reactions

Q13. The recommended action when a patient develops a severe hypersensitivity rash on cotrimoxazole is:

• Discontinue drug immediately and avoid future sulfonamides
• Reduce dose and continue therapy under observation
• Switch to higher dose to overcome resistance
• Add antihistamines and continue the same regimen

Correct Answer: Discontinue drug immediately and avoid future sulfonamides

Q14. Which infectious indication is cotrimoxazole NOT typically first-line for?

• Pseudomonas aeruginosa bloodstream infections
• Uncomplicated urinary tract infections by E. coli (where susceptible)
• Treatment and prophylaxis of Pneumocystis jirovecii pneumonia
• Treatment of Nocardia infections

Correct Answer: Pseudomonas aeruginosa bloodstream infections

Q15. What explains trimethoprim’s potentiation of the effect of angiotensin-converting enzyme inhibitors on potassium?

• Trimethoprim reduces renal potassium excretion leading to additive hyperkalemia
• Trimethoprim increases aldosterone secretion causing hypokalemia
• Trimethoprim induces CYP enzymes lowering ACE inhibitor levels
• Trimethoprim causes sodium retention counteracting ACE inhibitors

Correct Answer: Trimethoprim reduces renal potassium excretion leading to additive hyperkalemia

Q16. During high-dose sulfamethoxazole therapy, a preventive measure to reduce crystalluria is:

• Ensure adequate hydration and urine alkalinization
• Limit fluid intake to concentrate urine
• Administer with dairy products
• Avoid urine alkalinization to keep drug soluble

Correct Answer: Ensure adequate hydration and urine alkalinization

Q17. Which statement about cotrimoxazole’s effect on folate is correct?

• Trimethoprim inhibits dihydrofolate reductase, impairing tetrahydrofolate regeneration
• Sulfamethoxazole acts as a folate receptor agonist increasing folate uptake
• Trimethoprim enhances folate synthesis, preventing anemia
• Sulfamethoxazole converts folate to inactive metabolites

Correct Answer: Trimethoprim inhibits dihydrofolate reductase, impairing tetrahydrofolate regeneration

Q18. In patients with glucose-6-phosphate dehydrogenase (G6PD) deficiency, cotrimoxazole may cause:

• Hemolytic anemia due to oxidative stress from sulfonamides
• Thrombocytosis due to bone marrow stimulation
• Hyperbilirubinemia from increased conjugation
• Severe hypoglycemia from increased insulin release

Correct Answer: Hemolytic anemia due to oxidative stress from sulfonamides

Q19. Which pharmacodynamic classification best describes cotrimoxazole when the two agents are combined?

• Synergistic bactericidal acting at sequential targets
• Antagonistic bacteriostatic pair
• Additive antifungal therapy
• Independent effects with no interaction

Correct Answer: Synergistic bactericidal acting at sequential targets

Q20. A common dosing regimen for oral co-trimoxazole tablet labeled “DS” (double strength) contains which amounts?

• Trimethoprim 160 mg and sulfamethoxazole 800 mg
• Trimethoprim 80 mg and sulfamethoxazole 400 mg
• Trimethoprim 320 mg and sulfamethoxazole 1600 mg
• Trimethoprim 200 mg and sulfamethoxazole 400 mg

Correct Answer: Trimethoprim 160 mg and sulfamethoxazole 800 mg

Q21. Which metabolic interaction increases risk of severe neutropenia when cotrimoxazole is used with methotrexate?

• Both inhibit folate pathways leading to additive myelosuppression
• Cotrimoxazole induces methotrexate metabolism reducing toxicity
• Methotrexate increases renal clearance of cotrimoxazole
• No interaction; they act on unrelated pathways

Correct Answer: Both inhibit folate pathways leading to additive myelosuppression

Q22. Which laboratory sign may indicate trimethoprim-induced hyperkalemia?

• Elevated serum potassium concentration on basic metabolic panel
• Low serum calcium with normal potassium
• Marked leukocytosis with left shift
• Decreased serum creatinine clearance only

Correct Answer: Elevated serum potassium concentration on basic metabolic panel

Q23. Resistance to trimethoprim commonly arises from which bacterial change?

• Production of an altered dihydrofolate reductase with decreased drug affinity
• Overexpression of dihydropteroate synthase
• Increased synthesis of peptidoglycan
• Mutation in 16S rRNA

Correct Answer: Production of an altered dihydrofolate reductase with decreased drug affinity

Q24. For prophylaxis of Pneumocystis jirovecii pneumonia in HIV patients, cotrimoxazole dosing is typically:

• One double-strength tablet daily or three times weekly depending on risk
• High-dose IV therapy daily only
• Single low-dose tablet once monthly
• Topical administration to the respiratory tract

Correct Answer: One double-strength tablet daily or three times weekly depending on risk

Q25. A formulation consideration for pediatric cotrimoxazole includes:

• Oral suspension with weight-based dosing expressed as mg/kg of trimethoprim and sulfamethoxazole
• Use of double-strength tablets crushed in formula for neonates without dose calculation
• Avoiding any liquid formulation because of poor stability
• Administering only by intravenous infusion in children under 12

Correct Answer: Oral suspension with weight-based dosing expressed as mg/kg of trimethoprim and sulfamethoxazole

Q26. Which adverse effect is more likely with sulfamethoxazole than with trimethoprim?

• Photosensitivity and sulfa allergy manifestations
• Hyperkalemia due to potassium retention
• Folate deficiency anemia from DHFR inhibition
• Direct nephrotoxicity from trimethoprim metabolite

Correct Answer: Photosensitivity and sulfa allergy manifestations

Q27. When counseling a patient on cotrimoxazole for UTI, which statement is correct about its antibacterial spectrum?

• Active against many Enterobacteriaceae but not reliably against Pseudomonas
• Specifically more active against anaerobes than aerobes
• Primary agent for atypical intracellular organisms like Chlamydia only
• Ineffective against Gram-negative bacilli

Correct Answer: Active against many Enterobacteriaceae but not reliably against Pseudomonas

Q28. Which counselling point is important to reduce the risk of hypersensitivity with cotrimoxazole?

• Advise immediate discontinuation and medical review if rash or mucosal lesions occur
• Delay seeking care for mild rash as it usually resolves without action
• Increase dose to build tolerance gradually
• Take with grapefruit juice to reduce allergic risk

Correct Answer: Advise immediate discontinuation and medical review if rash or mucosal lesions occur

Q29. In severe community-acquired MRSA skin infections where cotrimoxazole is used, the typical rationale is:

• Oral activity against susceptible MRSA strains and good tissue penetration
• Universal MRSA resistance makes it ineffective
• Only intravenous formulations have MRSA activity
• It acts by inhibiting cell wall synthesis specific to MRSA

Correct Answer: Oral activity against susceptible MRSA strains and good tissue penetration

Q30. Which of the following is the best explanation for why folinic acid (leucovorin) might be given with high-dose trimethoprim therapy?

• To reduce bone marrow toxicity by bypassing inhibited DHFR and replenishing reduced folate pools
• To potentiate the antibacterial effect by increasing bacterial folate uptake
• To enhance renal excretion of trimethoprim
• To prevent sulfonamide crystalluria directly

Correct Answer: To reduce bone marrow toxicity by bypassing inhibited DHFR and replenishing reduced folate pools

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